Transducers have been used in a vast number of situations. Generally, a transducer is a device that converts one type of energy into another type of energy. Because direct measurement of a physical property can be difficult, a transducer is useful for converting measurement of a physical property that is difficult to measure to a physical property that is easier to measure.
One application for transducers is marine seismology. Marine seismology is the study of the subsurface of the Earth underneath bodies of water. Conventional measurements involve a device for wave generation at or near the surface that creates pressure waves aimed at the Earth's subsurface. The waves travel through the Earth's interior and the waves are both reflected and refracted as they progress through the subsurface. The pressure waves reflected from the subsurface are measured at a second device at or near the water's surface. The second device includes a transducer. Conventionally, the transducer is at least one hydrophone, and often more, that transduces the pressure waves into electrical signals. Some transducers available for measuring pressure waves include motion sensors that measure acoustic particle velocity or acceleration as a means for determining the pressure gradient associated with an acoustic wave. These sensors are disadvantageous because they measure any type of motion whether associated with an acoustic wave or another phenomenon. The other sources of motion add noise and error to the measurement obtained from the sensor.
The measurement process is complicated because the received signal is a combination of a reflected pressure wave from the air-water interface at the water surface and the reflected pressure wave from the subsurface of the Earth. Separating these signals can be accomplished if both the pressure and the pressure gradient are known for a given location. Conventional transducers such as hydrophones sense pressure. But sensors to sense pressure gradient are more difficult.
Transducers require a certain amount of time to respond to a physical property and generate an output. Transducers that respond very quickly are referred to as “fast response” transducers. Many hydrophones are fast response transducers in that they respond to pressure waves in an amount of time much shorter than the time of change in pressure caused by seismic waves. Therefore, hydrophones are often used to measure pressure changes over time at the location of the hydrophone. The hydrophone provides a measurement which is described to be a measurement of pressure as a function of time. The change in pressure with time may be referred to as a temporal derivative of pressure, or temporal gradient of pressure. However, the term “pressure gradient” as used herein is reserved exclusively to describe a change in pressure with a change in position. This usage is consistent with that used broadly in the art, in which case a hydrophone is a pressure sensor, and is not a pressure gradient sensor. A pressure gradient sensor provides a measurement of the change in pressure with position at the location of the sensor. Furthermore, a pressure gradient sensor may have fast response, in which case it provides a measurement of the change in pressure with position at the location of the sensor as a function of time.
Measuring gradients of a property are particularly challenging. One approach to measuring a gradient is to use multiple transducers to measure the desired property at multiple locations. The difference of the measurements made by the transducers may be divided by the distance between locations of the transducers. Assuming a constant gradient, this approach results in the rate of change of a property along the direction from one transducer to another transducer. Although simple, this approach has several problems. First, if the gradient varies between the location of two transducers (i.e., the first derivative is not constant), the measurement may not be sufficiently accurate. Second, difference measurements from two transducers can introduce problems including, but not limited to: relative position errors, common-mode rejection problems, and limitation of bandwidth and dynamic range compared to the individual transducer elements.
Other related material may be found in at least U.S. Pat. Nos. 7,239,577; 7,295,494; 7,245,954; 6,775,618; 3,715,713; U.S. Patent App. Pub. 20050194201; U.S. SIR Pub. No. H1524; and Acoustic Particle Velocity Sensors: Design, Performance, and Applications, Editors M. J. Berliner and J. F. Lindberg, AIP Conference Proceedings 368, September 1995, Woodbury, N.Y.: American Institute of Physics, 1996; Singh, Jasprit (2003) “Electronic and Optoelectronic Properties of Semiconductor Structures,” New York: Cambridge University Press; Chen, F. F. (1984) “Introduction to Plasma Physics and Controlled Fusion,” New York: Plenum Press; Smith, R. A., (1961) “Semiconductors,” New York: Cambridge University Press; Van Camp, P. E., Van Doren, V. E., Devreese, J. T. (1990) “Pressure dependence of the electronic properties of cubic III-IV In compounds,” Physical Review B, January 1990, pp. 1598-1602; and Data in Science and Technology, Editor in Chief: R. Poerschke, Semiconductors, Group IV Elements and III-V Compounds, edited by O. Madelung (Springer-Verlag, New York, 1991).